1) Technical Field of the Invention
This invention relates to a pressure switch with an airtight chamber partially defined by a diaphragm for electrically switching thereof in response to the stress applied to the diaphragm.
2) Description of Related Art
Some types of pressure switches have been so far proposed for a use of automobiles and industrial machines, in which a diaphragm of the pressure switch formed by partially thinning the semiconductor substrate is applied. Referring to FIGS. 16 through 18, the details of the conventional pressure switch disclosed in JPA06-267381, as an example, will be described hereinafter.
The conventional pressure switch 100 as shown in FIG. 16 basically comprises a silicon substrate 110 made of p-type single crystal and a glass substrate 130. The silicon substrate 110 includes, in its middle portion, a depression 111 formed on one surface (top surface), a recess 112 formed on the other surface (bottom surface) opposing to the depression 111, and a diaphragm 113 defined by and between the depression 111 and a recess 112 (with a thickness of several ten micrometers). The silicon substrate 110 further comprises a pair of p-type diffusion layers 114, 115, which are formed on the top surface, and spaced apart (electrically isolated) from each other through the depression 111. A pair of terminal electrode pads 116, 117 made of aluminum is also deposited on the top surface of the silicon substrate 110 for electrically connecting the pressure switch to the peripheral devices. A first wire layer 118 made of material such as aluminum is deposited on and extends along the diffusion layer 114 (left side), a side-wall, and a bottom of the depression 111.
On the other hand, the glass substrate 130 is joined on the top surface of the silicon substrate 110 so that an airtight chamber (reference pressure chamber) is defined between the depression 111 and the glass substrate 130. A second wire layer 131 also made of material such as aluminum is formed on a part of a bottom surface of the glass substrate 130 opposing the diffusion layer 115 (right side). The first and second wire layers 118, 131 are opposing each other within the airtight chamber 119, and each includes a contacting tip 120, 132 made of titanium, respectively. The diaphragm 113, when stressed, is deformed close to the glass substrate 130 so that the contacting tips 120, 132 contact each other so as to electrically connect the terminal electrode pads 116, 117 through the p-type diffusion layer 114, 115 and the wire layers 118, 131. Thus, the pressure switch can be switched in accordance with deformation (incurvature) of the diaphragm.
The silicon substrate 110 is designed to include a pair of offset paths 121, 133 pre-formed on the p-type diffusion layers 114, 115 for offsetting the thickness of the wire layers 118, 131 thereby to smoothen the joint surface where the silicon substrate 110 and the glass substrate 130 are joined together. In general, in order to achieve the high reliable pressure switch, the silicon substrate 110 and the glass substrate 130 should be hermetically sealed to define the airtight chamber 119, thereby maintaining its airtightness for a long time period.